<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    html,
    body {
      width: 100%;
      height: 100%;
      margin: 0;
      padding: 0;
    }

    .oDiv {
      width: 200px;
      height: 200px;
      background-color: red;
      position: absolute;
      top: 0;
      left: 0;
      right: 0;
      bottom: 0;
      margin: auto;
      /* position: relative;
      top: 50%;
      left: 50%;
      margin: -100px 0 0 -100px; */

      /* display: flex;
      vertical-align: center; */

    }
  </style>
</head>

<body>
  <div class="oDiv"></div>

</body>

</html>